Coated Metal Substrates That Are Susceptible to Wear, and Method for the Manufacture Thereof

ABSTRACT

The use of tungsten, molybdenum, tungsten or molybdenum alloys for coating metal substrates using a single-wire arc spraying method results in coatings that are characterized in particular by an increased Vickers hardness (VH).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application (under 35 USC § 371) of PCT/EP2020/063439, filed May 14, 2020, which claims benefit of German application No. 10 2019 112 586.3, filed May 14, 2019, the contents of each of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION Technical Field and State of the Art

The invention relates to coated metal substrates that are susceptible to wear, and to a method for their production. In particular, the invention relates to a modified filling chamber for a pressure die casting machine.

The inner surface of the filling chamber of a pressure die casting machine is most susceptible to wear in the area of its feed opening. Since hot casting material such as, for example, liquid aluminum, is fed mechanically through the feed opening, this material always strikes the same place on the inner surface of the filling chamber below the feed opening. After the filling chamber has been in use for a prolonged period of time, leaching can occur in the area below the filling opening, as a result of which the sliding movement of the pressure piston in the chamber might be hindered and the pressure piston is subjected to greater wear and tear. Moreover, in the case of vacuum pressure die casting, it becomes more difficult to reliably generate the requisite vacuum. For instance, German patent specification DE 42 29 338 C2 discloses a filling chamber that consists of a jacket element with a removable cylindrical insert. Here, the insert extends from the outer end of the filling chamber to axially below the feed opening and, at the inner end of the insert, it comes with its peripheral surface into contact with the inner wall of the filling chamber in a narrow ring area, whereas the outer end of the insert is guided coaxially relative to the filling chamber by a centering ring that engages between its outer circumference and the inner wall of the filling chamber. In this manner, a filling chamber is already created for pressure die casting machines whose main wear zone is replaceable directly on the pressure die casting machine. However, a longer service life would be desirable for such inserts.

Moreover, German patent specification DE 102 05 246 B4 discloses a filling chamber for a pressure die casting machine having a feed opening for liquid casting material in which a cooling means is provided in the filling chamber wall in the area opposite from the feed opening. The cooling means comprises a disc that can be inserted into the filling chamber wall from the outside and that is provided with at least one guide channel for a coolant. Among other things, this measure is intended to prolong the service life of a filling chamber insert.

European patent application EP 3 184 203 A1 likewise discloses a filling chamber for a pressure die casting machine whose cylindrical inner surface serves as a sliding surface for a pressure piston and which has a feed opening for liquid casting material as well as a removable cylindrical insert along whose inner surface the pressure piston slides and which is provided with a radial opening in the jacket filling that is connected to the feed opening of the filling chamber. Here, the inner surface of this removable insert consists at least partially of molybdenum or of a molybdenum alloy. The removable insert is made of a metal sheathing (steel) and of an internal bushing made of molybdenum or of a molybdenum alloy (Mo/Mo-alloy).

In the cited cases—in which a cylindrical insert is used to protect the feed opening or the filling chamber and in which it can be replaced with a new insert when wear and tear occurs—a relatively quick remedy is available in most of the commonly encountered cases of wear and tear of the sliding surface for the pressure piston.

International patent application WO 00/10752 A1 relates to a casting tool for casting molded parts made of non-ferrous metals (e.g. made of Al or Mg). Here, the casting mold is made of a heavy metal alloy (e.g. a tungsten alloy) or else their contact side facing the molded part is coated in a corresponding manner. For these purposes, the heavy metal alloys are used in the form of a sintered element.

U.S. Patent Appln. 2017/0266719 A1 discloses die casting systems and corresponding casting methods. Here, certain system parts are provided with coatings that react chemically with the metals that are to be cast (such as, for example, Al, Cu, Ti and their alloys) to a far lesser extent than the iron alloys used in the state of the art. However, the objective still exists to prolong the replacement intervals for the cited inserts or, generally speaking, the service life of a pressure die casting machine or of its most wear-prone part, namely, the filling chamber, in order to implement the casting process as efficiently as possible and without major interruptions. In general, there is also the objective of rendering surface coatings, especially on wear-prone metal substrates, mechanically and chemically more stable and thus more wear-resistant.

SUMMARY OF THE INVENTION

This objective is achieved according to the invention in that the inner surface of the filling chamber or the inner surface of an insert according to the invention for the filling chamber is entirely or partially coated with tungsten or a tungsten alloy. This also applies in general to wear-prone metal substrates.

It is known that the greatest wear and tear in a filling chamber occurs in its filling area. Thus, for example, in the case of aluminum pressure die casting, aluminum alloys that are low in iron attack the steel of the filling chamber or of an installed replaceable bushing, causing leaching especially in this area. In order to these drawbacks, the inner surface of the filling chamber, which is usually made of steel, is provided with tungsten or a tungsten alloy, preferably entirely but at least in the filling area. The invention likewise encompasses lining the inner surface of an insert for the filling chamber, which is preferably made of steel, with tungsten or a tungsten alloy, and this, in turn, can be done in its entirely or else partially.

Therefore, the present invention comprises a filling chamber for a pressure die casting machine whose cylindrical inner surface serves as a sliding surface for a pressure piston and which has a feed opening for liquid casting material such as, for example, aluminum in the case of an aluminum pressure die casting machine. In a preferred embodiment, the filling chamber has an insert that is preferably made of steel and that is provided with an opening in its circumferential surface that is connected to the feed opening of the filling chamber. Here, the inner surface of the filling chamber or the inner surface of the above-mentioned insert is entirely or partially provided or coated with molybdenum or of a molybdenum alloy, preferably in the filling area. The insert is about ¼ the length and about half the wall thickness of the filling chamber.

The use of tungsten or tungsten alloys for the above-mentioned applications is likewise encompassed by the invention.

In contrast to the insert described in European patent application EP 3 184 203 A1, the insert according to the invention is not a replaceable bushing in the form of a sleeve system in which a molybdenum insert is placed with a precise fit into an outer sleeve made of steel, but rather, the inner surface of the insert according to the invention, which is usually made of steel, is entirely or partially coated with tungsten or a tungsten alloy.

Except for the essential difference in terms of the durability of the insert and the type of coating, the filling chamber according to the invention corresponds to that of European patent application EP 3 184 203 A1. In this context, explicit reference is made to the filling chamber depicted there in FIG. 1 and to its description, for instance, in paragraph [0029], which can thus also serve as a description of a filling chamber according to the invention.

Tungsten or a tungsten alloy serves as the coating material for the inner surface of the filling chamber or of the insert according to the invention, whereby the inner surface of the filling chamber and the insert are preferably coated in their entirety. The alloy can be a binary, ternary or quaternary alloy or else an alloy with additional (metal) constituents. Examples of this are WNiFe alloys with preferably more than 50% by weight of tungsten (W) or WMoNiFe alloys in which the sum of tungsten and molybdenum is more than 50% by weight of the finished alloy.

Especially suitable alloys contain more than 50% by weight of tungsten. Particularly suitable forms for tungsten and tungsten alloys are wire and powder forms. The production then fundamentally makes use of powder metallurgical methods known to the person skilled in the art. Moreover, tungsten and corresponding tungsten alloys are available from various manufacturers and metal supply companies, among others, on the Internet.

The explanations given above about the preferred covering or coating material also apply to the coating of metal substrates in general.

It was found that, aside from the selection of a suitable coating material for creating chemically, thermally and mechanically stable coatings for filling chambers or on metal substrates, the coating method is likewise of considerable importance. In this context, it should be kept in mind that the linear thermal coefficients of expansion α of tungsten (α=4.5*10⁻⁶ 1/K at 20° C.) and steel (α=11-13*10⁻⁶ 1/K at 20° C.) differ markedly from each other, which makes it quite difficult to apply thermally and/or mechanically stable tungsten coatings onto a steel substrate.

The surface coating method of thermal spraying has proven to be particularly suitable for the purposes according to the invention. In the embodiment according to the invention, this method yields a very pronounced hardening of the injected layer of typically >800 HV Vickers hardness, whereas, for example, tungsten alloys otherwise only have a hardness between 280 HV and 400 HV. In addition to a marked improvement of the chemical/thermal and mechanical stability, the surface coatings applied by means of thermal spraying also stand out for excellent solvent resistance vis-à-vis liquid aluminum and are therefore preferably and particularly suited for use in aluminum pressure die casting systems.

The thermal spraying methods include the following:

-   -   arc spraying methods     -   wire flame spraying methods (or with rod)     -   powder flame spraying methods     -   high velocity spraying methods/high velocity flame spraying         methods (HVOF: high velocity oxy fuel)     -   laser spraying     -   cold spraying methods     -   detonation spraying     -   plasma spraying     -   PTA (plasma transferred arc).

Through the selection of the spraying method as the coating technology, very advantageous hardness values of the coating are attained for the application and these values are, in turn, decisive for attaining a substantial increase in the service life of the filling chambers, of inserts and of the metal substrates that are coated in this manner.

All of these methods can fundamentally be used for the purposes of the invention.

Thus, in addition to the tungsten-based material (filler material (spraying filler material)), the invention likewise relates to the coating method with which the material is applied. The methods of thermal spraying are surface coating methods. According to the standardized definition (DIN EN 657), filler materials, the so-called spraying filler materials, are melted off, melted on or melted down inside or outside of a spraying torch, then accelerated in a gas stream in the form of spraying particles, and cast onto the surface of the component that is to be coated. Here, the component surface (in contrast to cladding) is not melted and only slightly thermally stressed. A layer is formed since, when the spraying particles strike the component surface, they flatten to a greater or lesser extent depending on the process and on the material, primarily adhering due to mechanical clamping, and they build up the sprayed coating in layers. The quality characteristics of sprayed coatings are their low porosity, good adhesion to the component, absence of cracks and homogeneous microstructure. The achieved coating properties are substantially influenced by the temperature and the speed of the spraying particles at the point in time when they strike the surface that is to be coated. The surface condition (purity, activation, temperature) likewise exerts an influence on the quality characteristics such as the adhesive strength.

Electric arcs (arc spraying), plasma jet (plasma spraying), fuel-oxygen flame spraying or fuel-oxygen-high-velocity flame spraying (conventional and high-velocity flame spraying), fast preheated gases (cold-gas spraying) and laser beam (laser-beam spraying) serve as the energy carriers for melting on or melting down the spraying filler material. According to DIN standard EN 657, the spraying methods are classified on the basis of these criteria.

If the tungsten/tungsten alloy is used in the form of a wire, then the preferred method is wire arc spraying. Plasma spraying lends itself particularly well for pulverulent tungsten/tungsten alloys.

The above-mentioned methods of thermal spraying are known in the state of the art and thus also to the person skilled in the art; see, for example, http://www.gts-ev.de/html_d/ts-info.htm or the information brochure LINSPRAY®—“Gase and Know-how beim thermischen Spritzen” (“Gases and Know-how for Thermal Spraying”), Linde A G, Geschäftsbereich (Business Unit) Linde Gas, Unterschleissheim, Germany.

For the purposes of the present invention, it has been found that special representatives of coating methods which are generally known in the state of the art and which are to be appropriately modified in order to achieve the objective according to the invention lead to surprising results in terms of the quality and here, especially with an eye towards the achieved hardness, of the applied coatings.

Fundamentally, the thermal spraying methods and also the method for welding on the coating materials are preferred according to the invention. Here, the thermal spraying method offers a broad application spectrum for a wide array of materials that are to be applied.

In particular, among the thermal spraying methods, arc spraying is preferred according to the invention, being used in the so-called two-wire method and especially preferably in the so-called single-wire method. The preferred coating material is tungsten or tungsten alloys.

Additional objectives, advantages, features and application possibilities of the present invention can be gleaned from the description below of an embodiment making reference to the drawing. In this context, all of the described and/or depicted features, either on their own or in any meaningful combination, constitute the subject matter of the present invention, also irrespective of their compilation in the claims or in the claims to which they refer.

BRIEF DESCRIPTION OF THE DRAWING

The following is shown, at times schematically:

FIG. 1 is the spraying head of a single-wire device in schematic view.

DETAILED DESCRIPTION

Below, the preferred coating or spraying method will be described in greater detail by way of an example making reference to FIG. 1, without restricting the subject matter of the present invention in this process. Here, FIG. 1 shows the spraying head of a single-wire device in a schematic view. Here, the electrode 4 (tungsten electrode) is installed so as to be rotatable around the wire.

In the two-wire method as shown in FIG. 1, the arc is ignited between two wires, and in the single-wire method, the arc is ignited between the tungsten electrode 1 and the spraying wire 7. Here, the emitted electrons, that is to say, the arc, first pass through the shielding gas 2, in this case argon or an argon-hydrogen mixture, and subsequently through the gas mixture consisting of shielding gas 2 and atomizing gas 5 consisting of N₂ or air, thereby ionizing the gas into plasma 4 and heating it to approximately 15,000° C. The plasma 4 consists of a gas mixture consisting of shielding gas 1, in other words, argon or argon/hydrogen (Ar/ArH₂) together with atomizing gas 3 [sic, 5]. Exclusively shielding gas 2 remains flowing around the electrode 1, thus shielding it. In this process, the arc 4 melts the spraying wire 7 before the atomizing gas 5 atomizes the melted droplets of the spraying wire 7 at a very high pressure or at a very high speed. The atomized spraying wire with atomizing gas and shielding gas is designated by the reference numeral 8. The atomizing gas stream does not have to flow annularly around the shielding gas 2 as is the case with plasm welding, but rather, it can also be made to flow in through nozzles. In this case, the atomizing gas nozzle 6 shown in FIG. 1 would be eliminated and replaced by individual nozzles.

The spatial separation between the electrode 1 and the wire 7, whereby the electrode 1 can rotate around the wire 7, makes it possible to use less expensive nitrogen gas instead of noble gases. In this manner, an increase in hardness to approximately 600 HV can be attained in comparison to approximately 300 HV in the case of the starting material. It has been found that, with air as the atomizing gas 5, an additional and substantial increase in hardness to approximately 900 HV can be attained. Currents preferably in the range from 60 A to 159 A are used for single-wire spraying with wire made of tungsten or tungsten alloys.

The present invention especially translates into in a longer service life for the highly stressed filling chambers and thus also into a longer service life of a corresponding pressure die casting system, and the invention encompasses a coated filling chamber or a corresponding insert, a method for coating these components, the correspondingly equipped pressure die casting system and the use of tungsten or tungsten alloys for (entirely or partially) coating the filling chamber or the filling chamber insert. These statements apply correspondingly when it comes to metal substrates that are coated according to the invention.

In this context, the following must be pointed out:

tungsten/tungsten alloys are less brittle in comparison to molybdenum (Mo) or a molybdenum alloy and they are also less susceptible to oxidation. The thermal spraying that is preferred according to the invention makes it possible not only to protect an insert but also the entire filling chamber.

Owing to the spraying, the materials harden a great deal, as a result of which there is not only an improvement in terms of the dissolution caused by the liquid melt, but also at the same time, an improvement against mechanical wear and tear. Thus, the hardness of tungsten or of a typical tungsten alloy increases, for example, from 250-300 HV to more than 800 HV (e.g. 900 HV) for a sprayed-on coating (HV=Vickers hardness). Such hardening is remarkable and it does not occur, for example, with the iron or nickel alloys otherwise used. The method of thermal spraying also permits very simple repair of worn-out inserts of filling chambers. In this context, they do not need to have originally been provided with a coating.

Moreover, the following embodiments and application forms have been found which are likewise encompassed by the present invention.

Aside from the thermal spraying, the welding or the welding-on of a tungsten/tungsten alloy onto the substrate that is to be protected also brings about an improvement of the thermal, chemical and mechanical properties of the (coated) surface. The hardness of the welded-on tungsten/tungsten alloy is likewise markedly improved in comparison to the standard value, but generally does not reach the values that can be attained by thermal spraying. The welding-on is carried out with conventional welding processes such as TIG, MIG and MAG welding.

Thermal spraying or welding onto a substrate surface—as described for tungsten/tungsten alloys—also achieves a marked improvement of the thermal, chemical and mechanical properties (hardness) in the case of molybdenum or molybdenum alloys. For this reason, the coating of metal substrate surfaces by means of thermal spraying or welding-on of molybdenum or molybdenum alloys of the type described, for example, in European patent application EP 3 184 203 A1 (see paragraphs [0009] through [0012] there) and to which reference is hereby made explicitly for purposes of the disclosure, is likewise encompassed by the present invention for the purposes according to the invention.

Finally, the application according to the invention of tungsten/tungsten alloys or molybdenum of molybdenum alloys, whereby tungsten/tungsten alloys are preferred, is not limited only to the coating of the inner surfaces of filling chambers and filling chamber inserts of pressure die casting machines, but rather relates to all areas that are susceptible to wear and tear during casting, for example, of aluminum, zinc and copper as well as of the corresponding alloys. Therefore, the use of the coatings applied according to the invention encompasses all of the claimed components during the casting process and their use likewise encompasses all other conceivable wear and tear applications such as, for example, coatings for forming tools for aluminum, copper and steel, for camshafts, shunts, pistons and comparable areas of application. As set forth in the present invention, these are examples of metal substrates that are susceptible to wear.

LIST OF REFERENCE NUMERALS

1 cathode (tungsten electrode) 2 shielding gas argon or argon/hydrogen 3 shielding gas nozzle 4 arc (plasma consisting of shielding gas Ar/ArH₂) 5 atomizing gas (N₂ or air) 6 atomizing gas nozzle 7 spraying wire 8 atomized spraying wire (with atomizing gas and shielding gas) 

1.-7. (canceled)
 8. A metal substrate whose surface is entirely or partially coated with tungsten, molybdenum, tungsten alloys or molybdenum alloys applied by arc-spraying.
 9. The metal substrate according to claim 8, wherein the coated surface is an inner surface of a filling chamber or of a filling chamber insert of a pressure die casting machine.
 10. A pressure die casting machine, comprising the filling chamber or the filling chamber insert according to claim
 9. 11. The pressure die casting machine according to claim 10, wherein it is an aluminum pressure die casting machine.
 12. A method for coating a surface of a metal substrate, comprising: arc-spraying tungsten, molybdenum, a tungsten alloy or a molybdenum alloy onto the surface.
 13. (canceled)
 14. The method according to claim 12, wherein the arc spraying is a single-wire method.
 15. The method according to claim 12, wherein air is used as the atomizing gas.
 16. The method according to claim 12, wherein the tungsten alloy is an alloy containing more than 50% by weight of tungsten.
 17. The method according to claim 12, wherein the tungsten alloy is a WNiFe alloy with more than 50% by weight of tungsten or a WMoNiFe alloy in which the sum of the portions of tungsten and molybdenum amounts to more than 50% by weight.
 18. The method according to claim 12, wherein the molybdenum alloy is TZM molybdenum, consisting of 0.5% by weight of titanium, 0.08% by weight of zirconium, 0.01% to 0.04% by weight of carbon and the remainder consists of 100% by weight of molybdenum.
 19. The method according to claim 12, wherein the metal substrate comprises areas of a metal casting system that are susceptible to wear, or surfaces of forming tools, pistons, camshafts or shunts. 